1. Field of the Invention
The invention generally relates to television signals and in particular to a method and apparatus for nesting or encoding secondary signals, such as audio signals, within a television signal.
2. Description of Related Art
For certain television applications it is desirable to nest or encode secondary signals, such as audio signals, within a television signal. One such application involves interactive television toys wherein a teddy bear or other toy receives television signals provided by a VCR for display on a television set and extracts and responds to secondary signals nested within the television signal.
Typically, a control module is interconnected between the VCR and the television set. Preprogrammed video cassettes having television programs containing the nested signals are played using the VCR. The television signal is intercepted by the control module and nested audio signals, as well as various nested control signals, are extracted. The control module transmits the extracted signals to the teddy bear or other toy which includes internal mechanisms and circuitry for movement and speech. The teddy bear receives the extracted audio signals and plays back the audio signals while also moving in synchronization.
The entire television signal, including the nested audio signals, is transmitted from the control module into the television set which displays the prerecorded television program. The audio and control signals are nested or encoded within the television signal in a manner such that the television does not respond to, and is not affected by, the presence of the nested signals. Typically, the audio and control signals nested within the television signal are synchronized to portions of the television program such that the teddy bear or other toy appears to interact with characters and events displayed on the television set within the television program.
To implement such a system, the secondary voice and control signals are encoded within a portion of the television signal such that the secondary signals can be extracted from the television signal, yet such that the television itself does not respond to, and is not affected by, the presence of the secondary signals. A number of such techniques have been developed. Typically, portions of the television signal which do not provide video, audio or control signals to the television set are selected and secondary signals are encoded therein.
One example of such a technique is set forth in U.S. Pat. No. 4,665,431 to Cooper, wherein amplitude modulated audio samples are encoded on the "back porch" of a horizontal blank interval of a television signal. Although the method of Cooper may be effective for certain applications, it is not particularly effective for encoding signals within a television program recorded on a video cassette due to band-width limitations in consumer grade VCR's.
Another example of a technique for encoding signals within a television signal is set forth in U.S. Pat. No. 3,857,998 to Justice wherein secondary audio signals are encoded within a television signal using a chroma carrier of the television signal. In the technique of Justice, the secondary audio signals are encoded on a blanked portion of the back porch of the horizontal blanking interval. The vertical blanking interval of the television signal is not encoded. Because all of the lines of each video frame are not encoded with secondary audio signals, a problem arises in providing a continuous output of the secondary audio signal as the encoded signals are extracted from the television signal. To overcome this problem, and to provide a continuous audio output, all audio samples, except for those which would otherwise be nested within the lines containing the vertical blanking intervals, are delayed by a predetermined amount. Those signals which would otherwise be nested within the vertical blanking intervals are stored at different locations within the television signal and are not delayed. During the decoding of the secondary audio signals, delayed and non-delayed audio samples are recombined to recreate a continuous audio signal.
Although the method of Justice is useful for nesting secondary signals within a television program for playback on a VCR, it may not be easily applied to television signals which are broadcast because the secondary signal is stored at locations within the television signal which contains dedicated broadcast-related control signals during broadcast. Thus, the technique of Justice cannot be easily applied to providing nested signals for controlling the teddy bear or toys of the type described above wherein the television program is broadcast rather than stored on a video cassette. As can be appreciated, there is considerable advantage to providing such programs via broadcast television rather than on prerecorded video cassettes, with the greatest advantage perhaps being the diversity and variety of programming material that can be conveniently provided via broadcast television.
Another technique for encoded audio signals is described in U.S. Pat. No. 4,786,967 to Smith. In the technique of Smith, as with the technique of Justice, secondary audio samples are not stored within the vertical blanking lines. However, unlike Justice, the audio samples which would otherwise be stored within those vertical blanking lines are not stored elsewhere. A discontinuity results from the lack of audio samples stored in the vertical blanking signals. The discontinuity results in a 60-cycle hum during playback. Furthermore, higher-order harmonics based on the 60-cycle hum may also be present which further undesirably distort the output audio signal. With the technique of Smith, the 60-cycle hum problem is addressed by tapering the encoded audio signal to silence adjacent to the vertical blanking interval. In other words, rather than storing secondary audio samples at their full value throughout the horizontal portion of the signal, the magnitude of the audio signals are tapered at opposing ends of the horizontal portion such that the secondary audio signal is reduced to silence at the transition times from the display portion to the vertical blanking intervals. By tapering the audio signal, the 60-cycle hum generated during playback is reduced. Furthermore, the higher-order harmonics resulting from a sharp transition from no audio signal to a full audio signal are likewise reduced.
Although the technique of Smith may be effective in eliminating the 60-cycle hum problem without requiring the complicated delaying and recombination techniques of Justice, the technique of Smith may not be satisfactory for broadcast television signals. One reason that the technique of Smith may not be satisfactory for broadcast signals is that network and cable broadcasters impose limitations and constraints which prevent the encoding of audio signals within all of the video lines of the horizontal portion of the television signal. In particular, twenty-two lines of the video signal cannot be encoded due to such restrictions. The twenty-two lines which cannot be encoded represent over eight percent of the overall video signal. The lack of an audio signal within those twenty-two lines results in an audible 60-cycle hum and may further result in higher-order harmonics as well. Moreover, even if the technique of Smith were modified to also provide tapering in the vicinity of the twenty-two restricted lines, such a tapering would, at best, only be partially successful in eliminating the 60-cycle hum and higher-order harmonics.
Accordingly, it would be desirable to provide a technique for nesting secondary audio signals, as well as other secondary signals, within a television signal in a manner such that the encoded television signal may be broadcast, rather than merely provided by a video cassette recorder, and wherein 60-cycle hum and higher-order harmonic problems described above are substantially eliminated. The ability to provide broadcast television signals with nested audio and control signals is particularly desirable as a far greater variety of such programs can be provided conveniently to the customer than can be provided on video cassettes.
Although the audio nesting technique has been described above primarily with reference to the application of an interactive teddy bear or toy, encoded audio and control signals may be employed for a wide range of applications for controlling a wide range of devices, and are not limited to merely toys or other entertainment devices.